This invention relates to doping of direct band gap GaAs.sub.1-x P.sub.x semiconductor light emitting diodes with nitrogen for improving uniformity of optical power output and reducing degradation.
It has become desirable to employ electrographic or other photosensitive printers for text and graphics. In an electrographic printer, an electrostatic charge is formed on a photoreceptive surface of a moving drum or belt and selected areas of the surface are discharged by exposure to light. A printing toner is applied to the drum and adheres to the areas having an electrostatic charge and does not adhere to the discharged areas. A toner is then transferred to a sheet of plain paper and is heat fused to the paper. By controlling the areas illuminated and the areas not illuminated, characters, lines and other images may be produced on the paper.
One type of nonimpact printer employs an array of light emitting diodes (LEDs) for exposing the photoreceptive surface. A row, or two closely spaced rows, of minute LEDs are positioned near an elongated lens array so that their images are arrayed across the surface to be illuminated. As the surface moves past the line of LEDs, they are selectively activated to either emit light or not, thereby exposing or not exposing the photoreceptive surface in a pattern corresponding to the LEDs activated.
To form good images in an LED printer, it is desirable that all of the LEDs produce controlled light output power when activated. This assures a uniform quality image all the way across a paper for black-and-white printing and control of exposure for gray-scale printing. Typically, in gray-scale printing where the light output as a function of current is known, the ON-time of the LED is controlled to obtain a desired exposure of the electrographic surface.
Light emitting diodes for printheads are formed on wafers of GaAsP or the like suitably doped to conduct current and emit light at a diode junction. Long arrays of LEDs are formed on a wafer which is cut into separate chips, each having an array of LEDs. A row of such chips are assembled end-to-end on the printhead. It is desirable that the efficiency of the LEDs on a chip be reasonably uniform and that the efficiency degrade only a small and uniform amount as the LEDs are used. This is desirable for maintaining uniformity of illumination and good efficiency over the lifetime of the printer.
The amount of time that each LED has been on and its temperature are two factors which affect the amount of light output from each LED. It is believed that the degradation of light output power efficiency is related to the number of defects or dislocations at the junctions in the crystalline lattice structure of the doped semiconductor of the LED.
Light emitting diodes are made from both direct and indirect type semiconductors. In a direct type semi-conductor, the minimum in the conduction band is at the same crystal momentum as the maximum in the valence band. Thus, when an electron makes a transition from the valence band to the conduction band, there is no requirement for a change in crystal momentum. In an indirect semiconductor, on the other hand, the minimum in the conduction band is at a different crystal momentum from the maximum in the valence band. The electron transition requires a change in crystal momentum as well as the band gap energy. Thus, the probability of radiative recombination is reduced. Radiative recombination occurs readily in direct band gap semiconductor LEDs. On the other hand, in indirect band gap semiconductors, the probability of radiative recombination is rather low and the crystals are, therefore, doped to provide recombination centers for enhancing the radiative transitions.
In the gallium arsenic phosphorus compound system, GaAs.sub.1-x P.sub.x, when x is greater than 0.45, the semiconductor has an indirect band gap. Nitrogen is used to replace some of the phosphorus atoms in the crystal lattice. The outer electronic structure of nitrogen is similar to that of phosphorus, but the inner electronic structure is very different. This produces an isoelectronic recombination center close to the minimum of the conduction band and greatly enhances the probability of radiative recombination in this indirect band gap semiconductor.
Nitrogen doping has been avoided in direct band gap GaAs.sub.1-x P.sub.x since there is no need for additional recombination centers, and the quantum efficiency and light output power from the LED are reduced.
It remains desirable to minimize degradation in light output efficiency in a direct band gap semiconductor. It is proposed that pinning of dislocations and defects may make a more perfect crystal in an epitaxial layer of an LED and may also minimize migration or extension of dislocations to the light emitting junction, thereby enhancing light output efficiency and minimizing degradation.